Unit 2
Tests
This method is based on the principle, that a liquid penetrant is drawn into surface flaws by capillary action and subsequently revealed by developer materials in conjunction with a visual inspection. This method employs a penetrating liquid, applied over the cleaned surface of the component, which enters the discontinuities under capillaries action.
After an adequate time (also called dwell time), the excess penetrant is removed from the surface either by solvent or by water, depending upon the type of penetrant used.
The washed surface is dried and a thin layer of the developer (either fluffy talc powder or talc powder suspended in a volatile liquid) is applied uniformly over the surface. The developer act as a blotter and draws out any liquid remaining in the discontinuity. An indication is produced over the background of the developer layer when the discontinuities are open to the surface. The indications are examined either in daylight, adequate artificial illumination, or under black light (λ=3650), depending on the application of the coloured or fluorescent penetrant liquid.
The procedures cover the liquid penetrant examination by a solvent removable penetrant process using colour contrast technique for detecting surface discontinuities of base metal and weldment required by the applicable code and PO specifications.
1. Equipment used in DPI is as mentioned below-
- Penetrant /Emulsifier/Wet Developer station-Immersion Tank:
Immersion is the process normally used to apply penetrant emulsifier and wet developer to parts surface. Depending on part size, a tank of various sizes is used for the immersion process. Those tanks consist of a power immersion grille which is used to lower and raise large parts or baskets of a part in and out of the tank.
2. Rest/Drain Station-Roller Grilles:
Roller grilles support part during “rest” or “drain” cycles and facilitate the movement of heavy or basketed parts. These stations provide a support area equal to the basic tank size.
The Roller grille unit can be bolted to adjacent components or optical support legs may be used.
900 grille units accommodate the turning or reversal of the workflow.
3. Pre-rinse station:
The penetrant must be removed from the part surfaces so that even the smallest flaws are easily detected.
A pre-rinse station, immediately after penetrant applications stations, allows most of the penetrant to be flushed away from the surface of the part, thus relatively little penetrant is carried forward with the part into the penetrant remover station and remover life extended.
4. Remover station:
A hydraulic penetrant remover must be applied to the part following pre-rinse to assure complete removal of the surface penetrant.
Application is either by immersion or by spraying.
Immersion tanks are equipped with air agitators to achieve the scrubbing action necessary for the remover to be effective. Immersion is recommended for uniform coverage of parts having complex shapes.
A power immersion grille is often used to provide additional agitations.
5. Rinse Station:
All parts had to go through “final rinse” where all remaining penetrant, whether water wash, hydrophilic or lipophilic, is removed from the part’s surface.
A black-light is provided to ensure that proper washing is accomplished.
The final rinsing is done with a hand hose and spray nozzle. A hydro wash power spray nozzle is a frequently used option for faster cleaning when permitted by specification.
6. Tabletop Rinse Station:
It is used in place of the conventional rinse tanks, the tabletop rinse unit permits easy, manual access to hard rinse parts and to parts with entrapment areas that must be turned and emptied after rinsing.
Working at waist height reduces fatigue. A roller grille provides an ideal work platform. The tabletop unit includes a hose and spray nozzle and a portable black light for surface inspection. The hydro wash power spray is a popular option.
7. Dryer station:
All parts must be thoroughly dried during this processing. If dry developers are used, parts are dried before the application of the developer. If the wet developer is used, they are dried after the application of the developer.
The dryers provide uniform heating and air circulation throughout the drying chamber, with air intake to regulate humidity for optimum drying.
8. Inspection station:
Sturdy inspection booths provide the darkened area and the black lights for thorough fluorescent penetrant inspection.
Ample room is provided for handling and roller grilles facilitate the movement of heavy or basketed parts.
9. Dry developer Station:
The developer acts to pull the penetrant remaining in a cavity to the surface where it is readily visible under black light. The wet or dry developer may be used.
2. Techniques for Liquid Penetrant testing:
There are three major groups of the penetrant process:
- Water-soluble penetrant techniques
- Post-emulsifiable with water rinsing technique
- Solvent removable, the difference being in the method used to remove the excess penetrant.
Key Take away:
LPI is a method of detection of defects in which penetrant is drawn into surface flaws by capillary action and subsequently revealed by developer material in conjunction with a visual inspection.
- Penetrant /Emulsifier/Wet Developer station-Immersion Tank:
Immersion is the process normally used to apply penetrant emulsifier and wet developer to parts surface. Depending on part size, a tank of various sizes is used for the immersion process. Those tanks consist of a power immersion grille which is used to lower and raise large parts or baskets of a part in and out of the tank.
2. Rest/Drain Station-Roller Grilles:
Roller grilles support part during “rest” or “drain” cycles and facilitate the movement of heavy or basketed parts. These stations provide a support area equal to the basic tank size.
The Roller grille unit can be bolted to adjacent components or optical support legs may be used.
900 grille units accommodate the turning or reversal of the workflow.
3. Pre-rinse station:
The penetrant must be removed from the part surfaces so that even the smallest flaws are easily detected.
A pre-rinse station, immediately after penetrant applications stations, allows most of the penetrant to be flushed away from the surface of the part, thus relatively little penetrant is carried forward with the part into the penetrant remover station and remover life extended.
4. Remover station:
A hydraulic penetrant remover must be applied to the part following pre-rinse to assure complete removal of the surface penetrant.
Application is either by immersion or by spraying.
Immersion tanks are equipped with air agitators to achieve the scrubbing action necessary for the remover to be effective. Immersion is recommended for uniform coverage of parts having complex shapes.
A power immersion grille is often used to provide additional agitations.
5. Rinse Station:
All parts had to go through “final rinse” where all remaining penetrant, whether water wash, hydrophilic or lipophilic, is removed from the part’s surface.
A black-light is provided to ensure that proper washing is accomplished.
The final rinsing is done with a hand hose and spray nozzle. A hydro wash power spray nozzle is a frequently used option for faster cleaning when permitted by specification.
6. Tabletop Rinse Station:
It is used in place of the conventional rinse tanks, the tabletop rinse unit permits easy, manual access to hard rinse parts and to parts with entrapment areas that must be turned and emptied after rinsing.
Working at waist height reduces fatigue. A roller grille provides an ideal work platform. The tabletop unit includes a hose and spray nozzle and a portable black light for surface inspection. A hydro wash power spray is a popular option.
7. Dryer station:
All parts must be thoroughly dried during this processing. If dry developers are used, parts are dried before the application of the developer. If the wet developer is used, they are dried after the application of the developer.
The dryers provide uniform heating and air circulation throughout the drying chamber, with air intake to regulate humidity for optimum drying.
8. Inspection station:
Sturdy inspection booths provide the darkened area and the black lights for thorough fluorescent penetrant inspection.
Ample room is provided for handling and roller grilles facilitate the movement of heavy or basketed parts.
9. Dry developer Station:
The developer acts to pull the penetrant remaining in a cavity to the surface where it is readily visible under black light. The wet or dry developer may be used.
Dry developer stations are further classified as:
- Dynamic cloud station:
The air in the flat bottom chamber is pre-dried and circulated at low velocity to sustain a continuous powder cloud in which the parts are placed.
This method involves a sealed, fully enclosed dry powder, container to minimize waste, due to ‘caking:”
2. Swirl cloud station:
The swirl cloud station in an airtight chamber with an air manifold and hand valve.
Solid end doors provide entry and exit.
3. Developer Dip Station:
The tank for the dipping process has baffled ductwork for dust collector hook up.
4. Tabletop Developer station:
It is used in place of the developer dip station, the tabletop developer unit permits easy, manual application of the dry developer.
Advantages of Liquid penetrant test are given below:
- It is portable to use.
- Highly sensitive to small surface defects.
- Easy to inspect large areas and volumes.
- The low cost involved.
- Complex parts are inspected routinely.
- Proper visualization of defect is provided, as indications are directly produced on the surface of parts.
- It is free from any material limitations that is it can inspect any type of material that may or may not be magnetic, metallic, or conductive.
A penetrant is a liquid material, capable of wetting the entire surfaces and being drawn into the fine opening, is applied to the workpiece by dipping, spraying, or brushing.
In liquid penetrate inspection petroleum or water-based carrier fluid with fluorescent red colour dye is used as a penetrant.
Classifications of penetrant:
- Fluorescent Penetrant:
It consists of a dye that fluorescent when ultraviolet radiations are exposed to them.
It requires a darkened area and ultraviolet radiation for inspection.
These are very sensitive and more vulnerable to contamination.
2. Visible Penetrants:
It consists of red dye, which produces a fine range of contrast against the white developer background.
It does not need any dark area and ultraviolet radiation for inspection.
These are lesser sensitive and lesser vulnerable to contamination compared to fluorescent penetrant.
3. Water washable:
Water washable penetrants are also referred to as self-emulsifying penetrants.
This penetrant consists of an emulsifying agent namely detergents, which help it, get removed from the parts by rinsing with water only.
4. Post Emulsifiable (Lipophilic):
These are oil soluble and interact with an oil-based emulsifier to make removal possible.
5. Solvent removable:
These use solvents to penetrate removal possible.
6. Post-Emulsifiable (Hydrophilic):
These are water-soluble and interact with a detergent-based emulsifier to remove the excess penetrant with water wash only.
Key Takeaways:
Penetrant is a liquid material capable of wetting the entire surface and being drawn into a fine opening.
Fluorescent penetrant, washable penetrants, etc. are its classification.
The developer plays a vital role in penetrant testing as it pulls out the penetrant from the defect surface and spread it out over there, which helps in getting information of clear indication of flaw.
When it is exposed to ultraviolet radiations, it reflects and refracts the incident ultraviolet light, which causes brighter indications than the indications of penetrant only in presence of ultraviolet light.
Apart from these effects, it also produces a white background which in turn produces a fine contrast of indications.
Developers are classified as:
- Dry Powder:
Dry developers are a white powder that can be applied on the surface to be inspected in some of the following ways:
- By using electrostatic powder spray guns.
- By putting it into a dust cabinet.
- By using puffers to dust parts with powder.
- By dipping parts in a container of developers.
2. Water-soluble:’
These developers are consists of some chemical that are water-soluble and develops layer when water is evaporated.
Water-soluble developers may be applied by dipping, pouring, or brushing, but the method of application is by spraying it onto the surface to be inspected.
In these developers drying is a big issue that is if the developer is not dried, then additional removal of entrapped penetrant may lead.
3. Water Suspendable:
This consists of insoluble developers particles suspended in water. Similarly, as water-soluble developers are applied, it is also applied but with frequent stirring or agitation.
These developers require forced drying, so that indication could be clear as water suspendable provides a slightly translucent white coating which affects the sensitivity of indications from flaws.
4. Non-Aqueous:
These developers are volatile solvents and are applied generally by a spray gun fitted on an aerosol can.
These developers do not require forced drying as it consists of highly volatile solvents and it should be applied thoroughly so that it can produce a slightly translucent white coating.
Key Takeaway:
It is a material that draws the penetrant back to the surface.
It is a fluorescent penetrant process that is here the penetrant is capable of producing fluorescent under the projection of ultraviolet light.
It has various steps:
- Part Preparation:
For effective inspection, it is very important that the defects are open and must be cleaned properly before the application of penetrant. The success of penetrant to enter test depends upon the ability of dye penetrant to enter surface defects.
In preparation, the part must not only be free of contamination but anything which can clog surfaces defects like rust, grease, oil, wax, paint, scale, etc.
For this cleaner like caustic and acids are used to remove paints and plastic coatings, aqueous cleaners are used to remove oil.
One thing is to be firmly considered that the cleaning agents used to remove contaminations must also be removed usually by water. Before applying penetrant, we have to get ensured first that the part to be inspected is clean and completely dry.
b. Applications of penetrant:
After cleaning for a part, zyglo penetrant is applied by spraying, brushing, or immersing.
After application, a waiting time is required, for the penetrant action to take place.
This waiting is called dwell time and it depends on the material being inspected and the type of defect.
c. Removal of penetrant:
Following three methods are used for removing excess penetrant from the surface of the component:
- Water washing:
The water wash employs self-emulsifying penetrants. The excess penetrant was removed by simply washing with water.
2. Post Emulsifying:
It involves two steps of removal. First, the excess penetrant is treated with an emulsifier for stipulated periods and then water washing.
3. Solvent removing:
It is done in two stages. Initially, as much penetrant as possible is wiped from the test surface with a clean dry line-free cloth.
The second cleaning is done with a clean line-free cloth moistened with a solvent cleaner. It is not advisable to flush the surface with solvent.
d. Part drying:
This involves the drying of part with a hot air dryer at a temperature below 1600 F. Over drying is avoided as it can reduce the effectiveness of inspection.
Zyglo method involves the use of three types of developers:
- Dry powder developer
- Aqueous developer, and
- Non-aqueous developer
Key Takeaway:
It is a fluorescent penetrant process that is here the penetrant is capable of producing fluorescence under the projection of UV light.
A system performance check is typically required daily, at the reactivation of the system after maintenance or repairs, or any time the system is suspected of being out of control. System performance checks involve processing a test specimen with a known defect to determine the process will reveal discontinuities of the size required. The specimen must be processed following the same procedure used to process production.
The nature of the defect can have a large effect on the sensitivity of a liquid penetrant inspection. Sensitivity can be defined as the smallest defect that can be detected with a degree of reliability.
When a homogeneous ferromagnetic material is placed in a magnetic field, it gets magnetized and the magnetic field farms a continuous circuit from pole to pole through the material
If any surface or subsurface discontinuity is present, the magnetic field gets deflected and forms a leakage field.
If fine particles of magnetic material are applied on the surface of the test material, the leakage field attracts the particles which form an outline of discontinuity and indicate the location, size, shape, and extent of the discontinuity.
This method is very sensitive for locating fine surface and sub-surface defects.
It is based on the principle that ‘a flaw distorts the magnetic field that has been impressed upon the sample to be tested. This taste involves the contribution of two tests: Visual inspection and magnetic flux leakage testing.
Consider a bar magnet shown in figure1. It has magnetic fields in and around the magnet the place from where the magnetic line of force enters is termed as the South Pole and the magnetic line of force that exits is termed as the North Pole.
When it breaks over a certain length, a magnetic line of forces gets existed from the North Pole and again renters the magnet through the South Pole.
Due to crack information, other magnetic poles are generated which are opposite. This spread out of the magnetic field appears like a leak and thus is called a flux leakage field. And if a fined shaped ferromagnetic particle is spread over the crack, the particles get attracted and create and cluster at the poles at the edge of cracks.
This cluster of particles is much easier to be seen than the actual crack and this forms the basis of magnetic particle testing.
Scope of magnetic Particle testing:
This technique can be used in magnetic particle examination of forged steels, where it will produce consistent results upon which acceptance standards for steel can be made.
Alternating current is not permitted because its capability to detect the sub-surfaces discontinuities is very limited, therefore it unsuitable.
Only rectified full or half-wave alternating current and direct current shall be used as the source of magnetization.
Electromagnetic yokes with portable batteries are outdated as per as scope is concerned.
Key Takeaway:
MPT is based upon the principle that a flow distorts the magnetic field that has been impressed upon the sample to be tested. It is the contribution of two tests:
- Visual inspection
- Magnetic flux leakage
1. Ferromagnetic material:
Ferromagnetic materials usually contain domains of various sizes. The spin magnetic moments in each domain are generally aligned parallel to each other. As a result, each domain exhibits saturate magnetization in a particular direction that is parallel to the direction of spin moments. The vector sum of the magnetic moments from all the domains results in a non-zero magnetization and is commonly referred to as spontaneous magnetization.
Example: Fe, Co, and Ni, etc.
2. Non-ferromagnetic materials:
These are similar to Ferromagnetic materials but the spin moments in each domain are aligned opposite to each other as represented in the middle figure. Consequently, each domain results in a zero magnetization and hence a zero spontaneous magnetization. These are metal composites that do not have a magnetic property.
Examples: Chromium, FeMn alloy, and NiO, etc.
1. Equipment:
- A wet horizontal MPI machine is the most commonly used mass-production inspection machine.
- The machine has a head and tailstock where the part is placed to magnetize. In between head and tailstock is typically an induction coil, which is used to change the orientation of the magnetic field by ninety degrees from the headstock.
- Mobile power packs are custom built magnetizing power supplies used in wire wrapping applications.
- The magnetic yoke is a handheld device that induces a magnetic field between two poles.
2. Testing:
- Pre-clean the part from dust, rust, and scale, etc.
- Apply alternating current to produce the magnetic field around it. If a steel bar is placed within the magnetic field, a magnetic field will be produced a whose line of flux travel along.
- Sprinkle the magnetic particles either in dry or liquid suspends form.
- Distribution is viewed under proper light which should be kept up to illumination of 400-500 lux.
- Draining
- To make a record, the area is photographed
- Inspection
- Demagnetize and final cleaning operation completes the process.
1. Advantages:
- It is a very quick and simple test process.
- It is highly sensitive to the detection of surface and slightly sub-surface linear indications.
- It can detect both surfaces and near sub-surface defects.
- This method tends to automation and high volume production inspection.
- It is a very portable inspection method, especially when used with battery-powered equipment.
2. Limitation:
- It requires careful and proper cleaning of the test surface.
- It is not applicable for austenitic iron castings.
- It is most effective when flaws and current flow are perpendicular to each other.
- The empirical current level is critical to a successful inspection.
- It is an operator-dependent technique.
3. Interpretation of results:
- After application of Ultraviolet radiation while the operator looks for indications of defects that are 0 to ±45 degrees from the path of the current flows through the part.
- Indications only appear 45 to 90 degrees of the magnetic field applied. The easiest way to quickly figure out which way the magnetic field is running is to grab the part with either hand between headstocks laying your thumb against the part, this is called either left or right thumb rule.
- The direction of the thump point tells us the direction current is flowing, the magnetic field will be running 90 degrees from the current path.
- The part is either accepted or rejected based on predefined criteria
- The part is then demagnetized.
- Depending on requirements, the orientation of the magnetic field may need not to be changed 90 degrees to inspect for indications.
1. DC magnetization:
- DC produces a magnetic field that penetrates through the workpiece and is more sensitive than AC to detect the flaws.
- DC is used for the detection of both sub-surface and surface flaws/discontinuities. With DC it is possible to obtain full penetration into the object which is a major advantage.
- There are several ways to accomplish direct magnetization.
- One method involves clamping the component between two electrical contacts in a special piece of equipment.
- Current is passed through the component and a circular magnetic field is established in and around the component.
- When the magnetizing current is stopped, a residual magnetic field will remain within the component.
- The strength of the induced magnetic field is proportional to the amount of current passed through the component.
2. AC magnetization:
- In this method, only the surface of the metal is magnetized.
- It provides maximum flux density in detecting surface discontinuities and cracks.
- It is comparatively easy to demagnetize. Particle mobility is better but it is not effective in the case of sub-surface discontinuities.
In Magnetic particle testing, DC and AC can be used for producing the magnetic field. The field produced by DC penetrates the cross-section of parts but in the case of AC field is confined to metal or near the surface of the part.
Skin effect is the tendency of an alternating electric current to distribute itself within a conductor with the current density being largest near the surface of the conductor, decreasing at greater depths.
The skin effect causes the effective resistance of the conductor to increase at higher frequencies where the skin depth is smaller, thus reducing the effective cross-section of the conductor.
The skin effect is due to opposing eddy current induced by changing the magnetic field resulting from the alternating current.
- Dry Magnetic powders:
They can be used in red, black, gray, yellow, and several other colours so that a high level of contrast between the particles and the part being inspected can be achieved.
The size of the magnetic particle is also very important. Dry magnetic particles are produced to include a range of particle sizes.
In the case of the dry method, dry magnetic powder is sprinkled on the surface of the component. The fine particle is around (0.002 inches) in size and is about three times smaller in diameter and 20 times lighter than the coarse particle (0.006 inches).
It is more useful on a rough surface and provides the greatest probability of inspection.
2. Wet Magnetic powder:
The wet method is generally used for samples, processed on a horizontal wet type testing unit. The wet magnetic particle testing method is generally more sensitive than the dry because the suspension provides the particles with more sensitivity than the dry. After all, the suspension provides the particles with more mobility and makes it possible for smaller particles with more mobility and makes it possible for smaller particles to be used since dust and adherence to surface contamination is reduced or eliminated. The particles used with the wet method are smaller in size than those used in the dry method.
The particles are typically 0.0004 inches in size and the synthetic iron oxides have particle diameters around 0.000004 inches. This very small size is a result of the process used to form the particles and is not particularly desirable, as the particles are almost too settle out of suspension.
1. Direct Method:
a. By Direct Component Magnetization:
In this, a current is directly passed through the component whereas according to the right-hand clasp rule, a magnetic field will produce, which is known as the magnetic line of flux, and it is normal to the direction of current and forms a circular field in an around the conductor.
In this method, good electrical contact must be maintained
b. Circular Magnetization:
It involves the clamping of the component between two electrical contacts.
A magnetic field transverse to the long axis of the workpiece is produced, when the magnetizing current is passed through the components axis.
When the magnetizing current is stopped, a residual magnetic field will remain within the component.
The strength of the induced magnetic field is proportional to the amount of current passed through the component.
2. Indirect Magnetization:
It is accomplished by using a strong external magnetic field to establish a magnetic field within the component.
- Longitudinal Magnetization:
The magnetic field is produced in a direction parallel to the axis of the component by placing it in a coil excited by an electric current.
The component becomes the core of an electromagnet and is magnetized by induction from the magnetic field created in the coil.
This method is also called central conductor magnetization.
b. By Electromagnets:
An electromagnet in form of an adjustable horse-shoe magnet called a yoke is used.
When it is placed on the component, a magnetic field flow between the north and south poles of the magnet is produced. By the use of coils and solenoids, the ferromagnetic materials can also be magnetized by induction techniques of indirect magnetization.
Key Take away:
There are various methods of magnetization of ferromagnetic materials which are economically classified under two headings:
Direct and Indirect magnetization
1. It is used to detect the porosity, seems, undercut, and cracks in the field of welding.
2. It is used for quality checks in Ni, Co alloys, Cast iron, etc.
3. Steel and iron industries.
4. Aircraft industries (landing gears etc. are checked).
References:
- Dr. D Vijay Kumar, A textbook on Non- destructive testing.
- Google websites.
- Palash Awasthi, notes on NDT
